1. Field of the Invention
The present invention relates to a device for measuring the performance of an optical member, and more particularly, to a device for measuring the wavefront aberration of a projection optical system that projects a pattern image of a reticle onto a substrate.
2. Description of the Related Art
Projection exposure apparatuses have been used in which a photosensitive substrate is exposed with a pattern provided on a reticle (photomask) when devices, such as semiconductor devices or liquid crystal display elements, are manufactured by photolithography. It is important that the projection exposure apparatuses use a projection optical system that has high imaging performance and provides little aberration, because the pattern on the reticle is required to be precisely transferred onto the photosensitive substrate at a predetermined reduction ratio. Particularly, transfer patterns have become more sensitive to aberration of the optical system in order to meet recent demands for finer semiconductor devices. For this reason, it is necessary to precisely measure optical performance (e.g., wavefront aberration) of the projection optical system. Further, it is important to simplify and speed up the measurement and to reduce the cost from the viewpoints of productivity and economic efficiency.
In a known measuring method, a pattern of a reticle is actually transferred onto a wafer, and a resist image of the pattern is observed and inspected with a scanning electron microscope. However, the measuring time is long, and inspection reproducibility is low because of errors resulting from resist application and development. In order to overcome these problems, a point diffraction interferometer (PDI) including a pinhole for forming an ideal spherical wave and a shearing interferometer (or a Talbot interferometer) using shearing interference have been proposed. Further, a line diffraction interferometer (LDI) has been recently proposed which has a slit for forming an ideal spherical wave only in a one-dimensional direction (for example, see Japanese Patent Laid-Open Nos. 2000-146705 and 2000-097666).
In a measuring device that measures the wavefront aberration with a PDI, a mask having a pinhole for forming light having an ideal spherical wavefront as a reference wavefront, and a window, through which light having a wavefront containing aberration information about an optical system to be measured passes, is placed behind the optical system. An image of an interference pattern (interference fringes) produced between the wavefront of light passing through the pinhole and the wavefront of light passing through the window is captured by an imaging element, and is processed to calculate a wavefront aberration of the optical system. In a measuring device that measures the wavefront aberration with an LDI, a mask having a slit for forming light having an ideal spherical wavefront only in a one-dimensional direction as a reference wavefront, and a window, through which light having a wavefront containing aberration information about an optical system to be measured passes, is placed behind the optical system. An image of an interference pattern (interference fringes) produced between the wavefront of light from the slit and the wavefront of light from the window is captured by an imaging element, and is processed to calculate a wavefront aberration of the optical system.
In these measuring devices for measuring a wavefront aberration, in general, the ideal wavefront is spherical, or spherical only in a one-dimensional direction, and a light-receiving surface of the imaging element for detecting the wavefront is flat. Therefore, the wavefront aberration obtained by analysis contains not only aberration of the optical system, but also an aberration component resulting from the detection of the spherical wavefront with such an imaging element. In order to remove the aberration component resulting from the spherical wavefront, for example, it is conceivable to place a Fourier transform lens directly before the light-receiving surface of the imaging element so that substantially parallel light enters the light-receiving surface, or to subject the interference pattern to specific processing.
In these methods, however, an additional aberration component is sometimes produced by manufacturing error of the Fourier transform lens or precision error in image processing (e.g., precision error in determination of a processing area).
A mask having a pinhole (or a slit) and a window is obtained, for example, by forming a chromium film on a transparent substrate made of quartz. However, it is difficult to remove an aberration component resulting from the thickness of the substrate, an aberration component resulting from the displacement (particularly in the height direction) of the imaging element, and an aberration component resulting from manufacturing error of the interferometer. In particular, when the wavefront aberration of a projection optical system having a high numerical aperture (NA) is measured precisely, an aberration component resulting from the manufacturing error of the interferometer is increased.